Pneumatic radial tires

Abstract: A pneumatic radial tire (1) has a double crown structure such that an annular recess portion (7) is arranged in the tread (6) at a given region in the widthwise direction thereof, in which the tread is divided into a central region (8), a middle region (9) and a side region (10), and at least one auxiliary reinforcing layer (5) is arranged on a belt (4) to as to substantially envelop a crown portion of the carcass (2) and the full outer surface of the belt, provided that a stretching ratio of cord in the auxiliary reinforcing cord layer (5) at positions corresponding to the central region (8) and side region (10) is equal to or greater than that at a position corresponding to the middle region (9), whereby the high-speed durability is improved together with the drainage property while controlling the increase of tire weight and cost as far as possible.

Description: This invention relates to a pneumatic radial tire having an improved high-speed durability while controlling increases of tire weight and cost as far as possible and having a tread in which an annular recess portion is arranged in the tread at a given region in the widthwise direction thereof. In general, it is known that a pneumatic radial tire having an annular recess portion in the widthwise center of the tread has a particularly good drainage property during running at high speeds. Since the drainage property improves as the volume of the annular recess portion becomes larger, the annular recess portion in the tire is preferably made as deep as possible, but it is restricted by the thickness of the tread. For this end, it is advantageous to make the depth of the annular recess portion deeper by depressing regions of the carcass and the belt at a position corresponding to the annular recess portion in the tread inward in the radial direction of the tire and along the circumference of the tread. There are also known pneumatic radial tires satisfying such a requirement. In the above conventional radial tires, the ends and central portion of the belt are apt to expand in the radial direction of the tire through centrifugal force during running at high speeds, and hence there is a danger of generating belt separation by the interaction between the above radial expansion and deformation produced during the running under load to this degrade the high-speed durability. Therefore, a cap layer covering the full outer surface of the belt, a narrow-width layer covering only the end portion of the belt, an auxiliary belt layer covering only the central portion of the belt or the like should be arranged as a belt reinforcing member, which increases the tire weight by the amount of the belt reinforcing member used as compared with tires having an ordinary belt structure to thus degrade the rolling resistance and fuel consumption. Thus, in the conventional radial tires it is desirable to improve the high-speed durability without increasing the tire weight and cost. Attention is also drawn to the disclosures of EP-A-0 465 786 and EP-A-0 319 588. It is, therefore, an aim of the present invention to give improved high-speed durability to a pneumatic radial tire having a tread in which an annular recess portion is arranged in the tread at a given region in the widthwise direction thereof for developing good drainage property while controlling the increase of tire weight and cost as far as possible. According to the invention, there is provided a pneumatic radial tire comprising a carcass comprised of at least one rubberized cord ply, a belt superimposed on the carcass and comprised of rubberized cord layers, the cords of which layers being crossed with each other, at least one auxiliary reinforcing cord layer substantially enveloping a crown portion of the carcass and the full outer surface of the belt, and a tread having an annular recess portion arranged in the tread at a given region in the widthwise direction thereof and depressed inward in the radial direction of the tire along the circumference of the tread together with the carcass and the belt, in which a section profile of the tread in the widthwise direction is divided into a central region extending between inflection points of the annular recess portion having an outwardly concave curvature in the radial direction of the tire, an intermediate region extending outward from each of the inflection points in the widthwise direction along an outwardly convex curvature in the radial direction to at least a position corresponding to a maximum diameter of the tire, and a side region extending outward from the intermediate region in the widthwise direction toward a side end of the tread, and wherein a stretching ratio of cord in the auxiliary reinforcing cord layer at positions corresponding to the central region and side region of the tread is equal to or greater than that at a position corresponding to the intermediate region of the tread. In a preferred embodiment of the invention, the intermediate region of the tread has a radius of curvature larger than those of the central region and the side region. The invention will be further described, by way of example only, with reference to the accompanying drawing. Fig. 1 is a diagrammatically sectional view in the widthwise direction of an embodiment of a pneumatic radial tire according to the invention. In Fig. 1 is sectionally shown an embodiment of a pneumatic radial tire according to the invention, in which numeral 1 is a tire, numeral 2 a carcass, numeral 3 a rubberized cord layer, numeral 4 a belt, numeral 5 an auxiliary reinforcing cord layer, numeral 6 a tread, numeral 7 an annular recess portion, numeral 8 a central region, numeral 9 an intermediate region, numeral 10 a side region and numeral 11 a ground contact width of the tread. The term "ground contact width of tread" used herein means a width of the tread contacting with the ground under usually used internal pressure and load. Moreover, such internal pressure and load can be naturally determined in accordance with the standard of the tire to be used. In the illustrated embodiment, the pneumatic radial tire 1 comprises the carcass 2 of one rubberized cord ply containing cords arranged at a cord angle of 90° with respect to the equatorial plane of the tire, the belt 4 superimposed on the carcass 2 and comprised of two rubberized cord layers 3, the cords of which layers being crossed with each other at a given cord angle with respect to the equatorial plane of the tire, one auxiliary reinforcing cord layer 5 substantially enveloping a crown portion of the carcass and the full outer surface of the belt 4 and containing heat-shrinkable cords arranged substantially in parallel to the equatorial plane of the tire, and the tread 6 reinforced with the belt 4 and the auxiliary reinforcing cord layer 5. The tread 6 has such a double crown structure that the central portion in the widthwise direction of the tread is concavely depressed inward in the radial direction of the tire together with the carcass 2 and the belt 4 along the circumference of the tire to form an annular recess portion 7, in which the section profile of the tread 6 in the widthwise direction thereof is divided into the central region 8 extending between inflection points of the annular recess portion 7 having an outwardly concave curvature in the radial direction of the tire, intermediate regions 9 extending outward from each of the inflection points in the widthwise direction along an outwardly convex curvature in the radial direction to at least a position corresponding to a maximum diameter of the tire and side regions 10 extending outward from each of the intermediate regions 9 in the widthwise direction toward the side ends of the tread. Moreover, the center region 8, intermediate region 9 and side region 10 somewhat change in accordance with the shape of the double crown structure. If the deepest point of the annular recess portion 7 locates at a center of the tread in the widthwise direction thereof, it is preferable that the central region 8 between the inflection points corresponds to 20-35% of the ground contact width 11 and the outward end of the intermediate region 9 in the widthwise direction is located in a position corresponding to 70-85% of the ground contact width 11 and the outward end of the side region 10 is located in a position corresponding to 100-105% of the ground contact width 11. Moreover, the position of the deepest point of the annular recess portion may be somewhat shifted from the center of the tread toward either end of the tread in the widthwise direction thereof. That is, when the ground contact width of the tread is divided into three equal parts, the deepest point of the annular recess portion locates within a central part including the tread center among the above three equal parts. According to the invention, the stretching ratio of the cord in the auxiliary reinforcing cord layer 5 at positions corresponding to the central region 8 and the side region 10 of the tread 6 is equal to or greater than that at a position corresponding to the intermediate region 9. The term "stretching ratio of cord in the auxiliary reinforcing cord layer" used herein means a stretching percentage when the cords of the auxiliary reinforcing cord layer before the winding on the drum are stretched by winding under tension or by expanding in radial direction in the vulcanization build-up. When the stretching ratio of the cord in the auxiliary reinforcing cord layer exceeds 5%, the cord is too stretched and hence the rubber penetrability of the cord is degraded to degrade the adhesion between cord and rubber. Further, the difference in the stretching ratio of the cord between the position corresponding to the central region 8 and side region 10 and the position corresponding to the intermediate region 9 is preferably not more than 2.5% from a viewpoint of production problems. The number of the auxiliary reinforcing cord layers 5 used is preferably one in view of the tire weight and cost, but the number of the layers may be increased in view of the high-speed durability, if necessary. The stretching ratio of the auxiliary reinforcing cord layer is determined as follows. That is, in the case of tires having an ordinary single crown structure, the belt, auxiliary reinforcing cord layer and tread rubber are successively applied onto a flat belt-tread (BT) drum to prepare a ring-shaped member, which is fitted onto an outer periphery of a green case and expanded in the radial direction to form a green tire, and thereafter built up through vulcanization to form a tire product. In this case, the shaping quantity in the vulcanization is calculated to determine the stretching ratio. In the tire according to the invention, however, the tread 6 has the double crown structure as mentioned above, so that the tire product is formed in the same manner as described above except for the use of a drum having a wave form corresponding to the double crown structure, during which the shaping quantities of the regions 8, 9 and 10 in the vulcanization are calculated to determine the stretching ratio of the auxiliary reinforcing cord layer. Alternatively, when the belt, auxiliary reinforcing cord layer and tread rubber are applied onto the flat BT drum, tension applied to the cord in the auxiliary reinforcing cord layer is adjusted in the widthwise direction of the drum to determine the stretching ratio of the auxiliary reinforcing cord layer in the widthwise direction. In the tread 6, it is preferable that the radius of curvature in the intermediate region 9 is larger than those in the central region 8 and side region 10. In the carcass 2, the cords are preferably arranged substantially perpendicular to the equatorial plane of the tire. The belt 4 is formed by arranging at least two rubberized layers of high-elasticity cords so as to cross the cords of these layers at a relatively small cord angle, preferably 20-30°, with respect to the equatorial plane of the tire. The pneumatic radial tire comprising the tread of double crown structure has hitherto been manufactured as follows. That is, an innerliner and a radial carcass of at least one rubberized cord ply are applied onto a drum, and the ends of these members are wound around a bead core set on the end portion thereof to produce a green case. Separately, a belt, auxiliary reinforcing cord layer and tread rubber are applied onto a flat BT drum to form a belt-tread band, which is fitted onto the outer periphery of the above green case and expanded in the radial direction to form a green tire, and then built-up through vulcanization in a mold having a shape corresponding to a double crown structure of the tread. In this case, the belt-tread band is generally formed on the flat BT drum under conditions that the inner diameter of the band is substantially constant in the widthwise direction and also the tension applied is substantially constant in the widthwise direction. Therefore, when the belt-tread band is fitted onto the green case and built-up through vulcanization in the mold so as to form the tread of double crown structure, the expanding degree of the auxiliary reinforcing cord layer in the radial direction differs in the widthwise direction of the auxiliary reinforcing cord layer. That is, the expanding degree of the auxiliary reinforcing cord layer becomes large at a position corresponding to the intermediate region of the tread and small at positions corresponding to the central region and side region of the tread, so that the stretching percentage of the cord in the auxiliary reinforcing cord layer becomes large at the position corresponding to the intermediate region and small at the positions corresponding to the central region and side region as compared with the state of winding on the BT drum. As a result, the central region and side region of the tread, which are apt to expand in the radial direction through centrifugal force during high-speed running, are liable to be lacking in the necessary stretching ratio of the cord. In the tires manufactured by the above conventional method, however, a member reinforcing the belt should be arranged at a position corresponding to the central region and side region of the tread in order to ensure high-speed durability. According to the invention, the stretching ratio of at least one auxiliary reinforcing cord layer 5 enveloping substantially the crown portion of the carcass 2 and the full outer surface of the belt 4 is adjusted to an appropriate value in the widthwise direction of the belt. Specifically, the stretching ratio of the auxiliary reinforcing cord layer 5 at positions corresponding to the central region 8 and side region 10 of the tread is made equal to or greater than that at a position corresponding to the intermediate region 9 by controlling a winding tension of a rubberized ribbon containing plural heat shrinkable cords such as nylon cords or the like when the ribbon is helically wound on the BT drum to form the auxiliary reinforcing cord layer, or by helically winding the ribbon under a constant tension or a controlled tension in the widthwise direction when using a drum having a shape similar to the tread shape of double crown structure. Thus, the expansion of the tread 6 at the central region 8 and side region 10 in the radial direction of the tire is controlled when a large centrifugal force is applied to the tire during high-speed running. Alternatively, heat shrinkable rubberized bands each having a width corresponding to the width of each of the central region, intermediate region and side region of the tread may be wound on the BT drum with or without the control of tension. As a result, it is not required to arrange a narrow-width member at positions corresponding to the central region and side region of the tread, so that the belt structure becomes simple. Furthermore, the radius of curvature of the intermediate region 9 in the tread 6 is made larger than those of the central region 8 and side region 10, whereby interlaminar strain in the belt during the ground contacting of the intermediate region becomes small and the durability can be improved. The following examples are given in illustration of the invention and are not intended as limitations thereof. Invention tires 1 and 2 The invention tire shown in Fig. 1 has a tire size of 225/50R16 and comprises a radial carcass 2 of one rubberized cord ply containing cords arranged at a cord angle of 90° with respect to the equatorial plane of the tire, a belt 4 superimposed on the carcass 2 and comprised of two rubberized cord layers 3, the cords of which layers 3 being crossed with each other at a cord angle of 26° with respect to the equatorial plane of the tire, an auxiliary reinforcing cord layer 5 enveloping substantially the crown portion of the carcass 2 and the full outer surface of the belt 4 and containing heat shrinkable cords arranged substantially in parallel to the equatorial plane of the tire, and a tread 6. The tread 6 has such a double crown structure that the central portion of the tread 6 in the widthwise direction thereof is concavely depressed (7) together with the carcass 2 and the belt 4 inward in the radial direction of the tire along the circumference of the tread. Further, the section profile of the tread 6 in its widthwise direction is divided into a central region 8 between inflection points of the annular recess portion 7 having an outwardly concave curvature in the radial direction of the tire (corresponding to 33% of the ground contact width 11), an intermediate region 9 extending outward from each of the inflection points to a position corresponding to 80% of the ground contact width 11 in the widthwise direction, and a side region 10 extending outward from the outer end of each intermediate region 9 toward a tread end in the widthwise direction. In the tire of the above structure, the stretching ratio of cords in the auxiliary reinforcing cord layer 5 at positions corresponding to the above regions 8, 9 and 10 of the tread 6 is calculated as a stretching percentage of outer circumferential length after the vulcanization build-up when outer circumferential lengths of the auxiliary reinforcing cord layer at positions corresponding to the above regions are standard in the formation of the belt-tread band by applying the belt, auxiliary reinforcing cord layer and tread rubber onto a BT drum. In the invention tire 1, the rubberized ribbon containing nylon cords therein is helically wound on a flat BT drum while controlling the winding tension of the cord in the widthwise direction of the auxiliary reinforcing cord layer whereby the stretching percentage of the cords after the vulcanization build up is 4 % at all positions corresponding to the above regions. In the invention tire 2, the ribbon is helically wound on a BT drum having a shape similar to a tread shape of double crown structure under a condition that the winding tension of the ribbon at positions corresponding to the central region and side region of the tread is made larger than that at a position corresponding to the intermediate region, whereby the stretching percentage of the cords in the auxiliary reinforcing cord layer after the vulcanization build-up is 3.5% at positions corresponding to the central region and side region and is 2.0% at a position corresponding to the intermediate region. Moreover, the radius of curvature in the tread 6 is 30 mm at the central region 8, 300 mm in the intermediate region and 150 mm in the side region. Comparative tire This tire is the same as in the invention tire 1 except that the auxiliary reinforcing cord layer is applied onto a flat BT drum under a uniform tension in the widthwise direction of the drum together with the belt and tread rubber and a pair of narrow-width members are arranged on both end portions of the auxiliary reinforcing cord layer. Moreover, each of the narrow-width members contains nylon cords arranged at a cord angle of 0° with respect to the equatorial plane of the tire. Conventional tire This tire is the same as in the invention tire 1 except that the auxiliary reinforcing cord layer is applied onto a flat BT drum under a uniform tension in the widthwise direction of the drum together with the belt and tread rubber. Test method The high-speed durability is evaluated by a high-speed drum durability test in which the test tire inflated to an internal pressure of 2.5 kgf/cm 2 is run on a drum of 3 m in diameter under a load of 450 kgf at a speed of 150 km/hr for 20 minutes and then the running speed is raised at a rate of 10 km/hr every 20 minutes until the occurrence of tire breakage. The results are shown in Table 1 together with tire weight and cost. Moreover, numerical values in Table 1 are represented by an index on the basis that the conventional tire is 100. As the index value becomes large, the better the high-speed durability, while as the index value becomes small, the lesser the tire weight and cost. As seen from the results of Table 1, the invention tires have improved high-speed durability as Conventional tire Comparative Tire Invention tire 1 Invention tire 2 High-speed durability 100 110 110 113 Tire weight 100 103 100 100 Cost per tire 100 103 100 100 compared with the conventional tire though the tire weight and cost are the same as in the conventional tire. Furthermore, the invention tires have improved tire weight and cost as compared with the comparative tire though the high-speed durability is the same as in the comparative tire. Therefore, the invention tires are superior to the comparative and conventional tires. As mentioned above, according to the invention, the stretching ratio of at least one auxiliary reinforcing cord layer substantially enveloping the crown portion of the carcass and the full outer surface of the belt is adequately adjusted in the widthwise direction of the belt, or the stretching ratio of the auxiliary reinforcing cord layer at positions corresponding to the central region and side region of the tread is made equal to or greater than that at a position corresponding to the intermediate region of the tread, whereby the expansion of the tread at the central region and side region in the radial direction of the tire is controlled when a large centrifugal force is generated in the tire during high-speed running and hence the occurrence of belt separation can be prevented and the high-speed durability can be improved. Furthermore, it is not required to arrange a narrow-width member for reinforcing the belt at positions corresponding to the central region and side region of the tread, so that the belt structure becomes simple and the tire weight and cost are advantageously decreased. Also, when the radius of curvature at the intermediate region of the tread is made larger than those at the central region and side region of the tread, the interlaminar strain of the belt can be suppressed at the ground contact area (i.e. intermediate region) and hence the high-speed durability can be further improved. Moreover, the volume of the annular recess portion can sufficiently be ensured by controlling the expansion of the central region in the radial direction, whereby the drainage property of the tread can be attained even during high-speed running.

Claims: A pneumatic radial tire (1) comprising a carcass (2) comprised of at least one rubberised cord ply, a belt (4) superimposed on the carcass and comprised of rubberized cord layers (3), the cords of which layers being crossed with each other, at least one auxiliary reinforcing cord layer (5) substantially enveloping a crown portion of the carcass and the full outer surface of the belt, and a tread (6) having an annular recess portion (7) arranged in the tread at a given region in the widthwise direction thereof and depressed inward in the radial direction of the tire along the circumference of the tread together with the carcass and the belt, in which a section profile of the tread in the widthwise direction is divided into a central region (8) extending between inflection points of the annular recess portion having an outwardly concave curvature in the radial direction of the tire, an intermediate region (9) extending outward from each of the inflection points in the widthwise direction along an outwardly convex curvature in the radial direction to at least a position corresponding to a maximum diameter of the tire, and a side region (10) extending outward from the intermediate region (9) in the widthwise direction toward a side end of the tread, and wherein a stretching ratio of cord in the auxiliary reinforcing cord layer (5) at positions corresponding to the central region (8) and side region (10) of the tread is equal to or greater than that at a position corresponding to the intermediate region (9) of the tread. A pneumatic radial tire as claimed in claim 1, characterized in that the central region (8) corresponds to 20-35% of a ground contact width (11) of the tread (6), and an outward end of the intermediate region (9) in the widthwise direction is in a position corresponding to 70-85% of the ground contact width (11), and an outward end of the side region (10) in the widthwise direction is in a position corresponding to 100-105% of the ground contact width. A pneumatic radial tire as claimed in claim 1 or 2, characterized in that the intermediate region (9) of the tread has a radius of curvature larger than those of the central region (8) and side region (10) of the tread. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that a deepest point of the annular recess portion (7) is located within a central part including a widthwise center of the tread when the tread is divided into three equal parts. A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that the stretching ratio of cords in the auxiliary reinforcing cord layer (5) is not more than 5% at positions corresponding to the central, intermediate and side regions (8, 9, 10) of the tread. A pneumatic radial tire as claimed in claim 5, characterized in that a difference in the stretching ratio of cords in the auxiliary reinforcing cord layer (5) between a position corresponding to the central region (8) and side region (10) of the tread and a position corresponding to the intermediate region (9) of the tread is not more than 2.5%. A pneumatic radial tire as claimed in any of claims 1 to 6, characterized in that the belt (4) is comprised of two rubberized cord layers (3) and the cords of said layers are crossed with each other at a cord angle of 20-30° with respect to the equatorial plane of the tire. A pneumatic radial tire as claimed in any of claims 1 to 7, characterized in that the auxiliary reinforcing cord layer (5) is a rubberized layer of heat shrinkable cords arranged substantially in parallel to the equatorial plane of the tire.

Inventors: TSUDA TORU, TSUDA, TORU
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1994
Main classification: B60C3
IPC codes: B60C11, B60C3, B29D30, B60C9